Noble gas transport into the mantle facilitated by high solubility in amphibole

The chemical evolution of both the Earth's atmosphere and mantle can be traced using noble gases(1-5). Their abundance in mantle and atmosphere reflects a balance between the flux of noble gases from the Earth's interior through magmatism, and the recycling of noble gases from the atmosphere back into the mantle at subduction zones. The flux of noble gases back into the mantle has long been thought to be negligible(1). However, analyses of samples from the mantle now suggest that this recycling flux is more significant(6-8), but the responsible mechanisms are unclear. Here we present high-pressure experimental measurements that demonstrate high solubility of noble gases in amphibole, an important hydrous mineral in altered oceanic crust(9-11). Noble gas solubility correlates with the concentration of unoccupied A-sites, which consist of a pair of opposing tetrahedra rings. We conclude that A-sites are energetically favourable locations for noble gas dissolution in amphibole that could allow recycling of noble gases into the mantle by subduction of altered oceanic crust. As many hydrous minerals in subducting slabs, such as serpentine and chlorite, have lattice structures similar to the A-site in amphibole, we suggest that these minerals may provide even more significant recycling pathways(7,11).